The present invention relates to a device for induction heating of metallic strips of differing widths having one multicoil transverse field inductor both above and below the strip to be heated, whose coil axes are positioned vertically to the strip surface.
A device for induction heating of flat metallic stock, having at least two inductors which are assigned in pairs lying above and below the metal stock, is known from German Patent Application 3928629 A1. In this device, the iron cores of at least one inductor have zigzag or wave-shaped grooves in the transport direction of the stock, into which the conductors are inlaid. The adjustment of the inductor power to the respective strip width is performed essentially by switching off selected coil conductors. An essential disadvantage of this known device is that optimized edge heating of the strips cannot be ensured due to the wound coil conductor course, since for conductors which lie in the edge region of the strip, one part of the conductor is nearer to the edge region of the strip than the other part.
The object of the present invention is thus to implement a device of the type initially cited in such a way that the disadvantages of the known relevant device are avoided, so that in the event of varying stock widths, a uniform heating pattern is achieved over the respective width, and particularly in the edge regions, with simple construction of the inductors.
This object is achieved according to the present invention in a device of the type initially cited in that, for optimized edge heating of the strips, the inductors each comprise at least one inductor segment, which is constructed as a coil composite of multiple approximately rectangular coils, which predominantly extend transversely to the transport direction of the strip, the coils having differing, stepped transverse extensions and the coil having the highest transverse extension extending at most up to the lateral edges of the widest strip and the coil having the lowest transverse extension extending at most up to the lateral edges of the narrow strip. In addition, each inductor segment is connected to a circuit for defined clocking of its coils and each inductor segment below the strip is assigned an identical inductor segment above the strip.
Using the device above, operators of heating devices are capable of treating the greatest possible spectrum of stripsxe2x80x94particularly in regard to the strip width, but also in regard to the strip thickness and the material. Through the differing, stepped coils, which may be switched on in a targeted way, the energy consumption is optimized and a uniform heating pattern is achieved independently of the width of the strip used, with maximum temperature oscillations of xc2x115xc2x0 C. In this case, the typical heating temperatures are approximately 400xc2x0 C. for aluminum strips and approximately 500-600xc2x0 C. for brass strips. The defined clocking of the coil selected for the respective strip width particularly counteracts overheating of the strip edges and therefore prevents warping or other quality losses; in this case, at least one coil may also be switched on permanently within a coil composite in addition to the clocked coils. The coil conductors of the upper inductor segments are switched in the same direction as the coil conductors precisely or approximately opposite below the strip to build up a magnetic field which penetrates the strip uniformly. The division of the inductors into inductor segments and the simple construction of the segments by using approximately rectangular coils reduces the production costs and the susceptibility to breakdown. Should a breakdown nonetheless occur, the affected inductor segment may be replaced individually. A long standstill time and high repair costs are therefore avoided.
The device according to the present invention may further be implemented in such a way that an inductor comprises multiple inductor segments, which are positioned one behind another at intervals in the transport direction of the strip. If there is a lack of space in furnaces which are too short, the inductor segments may also, however, be positioned one directly behind another. Through a divided inductor, the possibility results of switching each segment individually and therefore introducing the respective power necessary separately. Therefore, for example, the segments at the beginning of the heating device, which must heat the still cold stock, may introduce a higher power than the following segments.
The device according to the present invention may further be implemented in such a way that each inductor segment is a coil composite of three to eight coils. A coil composite of three to eight coils per inductor segment is simple to construct and produce. The coils, which are stepped in their transverse extension, have graduations of 4 to 10 cm to each strip side. This distance is selected low enough so that a strip whose edge is not sufficiently heated by the coil lying next to it may be heated optimally by clocking multiple coils.
The device according to the present invention may further be implemented in such a way that the difference of the transverse extension of one coil to the transverse extension of the next smaller or larger coil is at least 50 mm and at most 200 mm. A coil composite stepped in this way allows the operator of a facility to treat strips of different widths. Therefore, he is not only fixed on one strip width, but may heat multiple commercially available strips. If high requirements are placed on the temperature precision, a coil composite having small transverse extension differences must be selected. If the operator wants to treat strips of a width which may not be optimally heated by the coil composite already used in the furnace, he may easily remove the segments in the device and, for example, replace them by segments having coils of smaller transverse extensions and/or transverse extension differences.
The device according to the present invention may further be implemented in such a way that a coil composite is constructed from multiple nesting coils of differing transverse extensions, the coils having a shared axis.
The device according to the present invention may further be implemented in such a way that the coils of a coil composite are placed offset in relation to one another in the transport direction of the strip.
The above arrangements are used for optimizing the temperature distribution, particularly in the edge region of the strip. At the same time, there is the possibility of incorporating inductor segments of differing embodiments within an inductor.
The device according to the present invention may further be implemented in such a way that the coil conductors are positioned above one another or next to one another within a conductor groove. It is additionally possible for only one coil conductor to be in a conductor groove.
The device according to the present invention may further be implemented in such a way that at least two coils per inductor segment, selected as a function of the strip width, are switched in a clocked way so that only one coil is switched on at a time. In this way, for example, 500 or 1000 switching operations per second may be achieved. Through clocking of the coils of this type, overheating of the strip is prevented, particularly in the edge region. However, it is also conceivable to leave one coil continuously switched on, while two other coils are switched in a clocked way. In borderline cases, it may also be advisable to only switch on one coil. The power is provided in this case by one or more converters. The original 100% power of the converter may be relayed via thyristors to the coils of an inductor segment in such a way that, for example, one coil is constantly supplied with 70% of the total power, while two further clocked coils are assigned to receive 10% and 20% of the power, respectively. There is the possibility of clocking the coils of each inductor segment individually within an inductor.
The device according to the present invention may further be implemented in such a way that the frequency and/or duration of the switching operations is variably adjustable for each coil. The use of different frequencies and switching durations for the coils may promote uniform heating over the strip width.
The device according to the present invention may further be implemented in such a way that a scanner is provided to determine the temperature profile over the strip width. Therefore, any unforeseen deviations of the temperature profile, due to defective coils, for example, may be established as rapidly as possible.
The device according to the present invention may further be implemented in such a way that a circuit is provided for automatic clocking of the selected coils by analyzing the temperature profile established by the scanner. In this way, deviations from the intended temperature value are detected immediately. The desired temperature profile may be reached again by changing the clocking.
The device according to the present invention may further be implemented in such a way that at least one upper inductor segment is positioned offset transversely to the transport direction of the strip in relation to the assigned lower inductor segment. In this way, balancing of the temperature profile may be optimized.
The device according to the present invention may further be implemented in such a way that the offset between the upper inductor segment and the assigned lower inductor segment is variably adjustable.
The device according to the present invention may further be implemented in such a way that at least some of the inductor segments are mounted replaceably in the device. In this way, the inductor segments may be removed individually and replaced in case of breakdown. This also applies if strips are to be treated which may not be ideally heated by the inductor segments currently in the furnace due to their width. The furnace may be retrofitted for other strip width ranges through rapid replacement of the inductor segments in this case. In addition, it is conceivable that in sufficiently long furnaces, inductor segments for heating narrower strips are positioned in the furnace before or after inductor segments for wider strips. With an embodiment of this type, the corresponding inductor segments may be switched on and the others may be switched off, for the treatment of wider strips, for example. Therefore, strips of two strip width ranges may be treated in one furnace.
The device according to the present invention may be provided for the purpose of heating strips having a width of at least 200 mm.
The device according to the present invention may furthermore be provided for the purpose of heating strips having a width of at most 2000 mm. In this case, in the event strips of different widths are used, the strip width range in a furnace is selected in such a way that the widest strip to be heated is twice or three times as wide as the narrow strip, for example, the width of the narrow strip is 400 mm and that of the widest strip is 800 and/or 1200 mm.
Finally, the device may be used for heating metallic strips made of aluminum, steel, copper, or brass.
In the following part of the description, embodiments of the device according to the present invention are described with reference to 8 figures.
FIG. 1 shows a schematic view of two inductor segments, positioned without offset in relation to one another, with strip to be heated,
FIG. 2 shows a schematic view of two inductor segments, positioned with offset in relation to one another, with strip to be heated,
FIG. 3 shows a section through coil conductor grooves and coil conductors of an inductor segment,
FIG. 4 shows a further section through coil conductor grooves and coil conductors of an inductor segment,
FIG. 5 shows a schematic illustration of an inductor segment having electrical connections, a scanner, and strip to be heated,
FIG. 6 shows a schematic illustration of two inductor segments positioned one behind the other in the transport direction,
FIG. 7 shows a further schematic illustration of two inductor segments positioned one behind the other in the transport direction, and
FIG. 8 shows a diagram of the temperature distribution over the strip width of an aluminum band for different coil clockings.